During high-frequency surgery, if the actual area of contact between the neutral electrode and the patients body is too small there is a risk of receiving burns from the neutral electrode due to the large current densities which then occur.
Devices for monitoring the application of neutral electrodes in high-frequency surgery are known from DE 32 39 640 C2 or EP 0 390 937 A1. The DE 32 39 640 C2 reference discloses such a device having an impedance sensor with a resonant circuit comprising a secondary coil of a transformer and HF input capacitors of preferably two partial electrode surfaces. The impedance sensor detects the transition impedances of two partial electrode surfaces connected in series with the patient's tissues by application of a patient auxiliary current, and is moderated by the transition impedances. If this resonance network is excited at its exact resonant frequency, the patient transition resistance R alone becomes visible. This operating point can be attained in principle by positive voltage feedback. However, it has been found that such arrangements are vulnerable to interference signals produced by HF generators. As such, a positive voltage feedback would have to be tapped behind a filter, the phase shift of which depends on the tolerances of the components used to construct the filter. This phase shift has a large impact on the resulting closed loop oscillation frequency of the system. The parallel resonant circuit is then no longer excited at its exact resonant frequency, which results in measurement errors. Furthermore, such a system is typically not very accurate due to the tolerances of the relatively large number of components involved.